1. Field of the Invention
The present invention relates generally to an automotive engine and more specifically to a diagnostic arrangement for an EGR (exhaust gas recirculation) system thereof.
2. Description of the Prior Art
In order to reduce the amount NOx emitted from an internal combustion engine, it is known to selectively recirculate a portion of the exhaust gases back from the exhaust system and to mix this portion with the air-fuel mixture in the induction system prior to its introduction into the combustion chamber(s) of the engine. This technique reduces the peak combustion temperature and attenuates the formation of nitrogen oxides.
However, in the event that the valve which controls the flow of exhaust gas through a conduit which interconnects the exhaust and induction systems malfunctions and remains constantly closed for example, the amount of NOx formed increases markedly. On the other hand, if the valve should malfunction in a manner to remain constantly open, an excess of exhaust gases is supplied into the induction system with the result that the engine surges and exhibits deteriorated performance.
Under the above circumstances, it is relatively easy for the vehicle user to determine whether a malfunction has occurred simply on the basis of the engine driving characteristics. However, if the conduit via which the exhaust gases are recirculated to the induction system becomes becomes soiled with soot and the like type carbonaceous deposits and partially restricts the flow of gas therethrough, the amount of NOx which is formed is increased by relatively small amounts and renders it is very difficult for the driver to detect this phenomenon simply from the engine performance characteristics.
In certain places such as the state of California (U.S.A.) which have rather strict engine emission standards, it has been proposed to implement regulations which obviate reliance on humanly perceivable operational characteristics and which require onboard diagnostic arrangements which monitor the operation of fuel injectors, oxygen sensors, catalytic converters and the like and alert the vehicle user to malfunctions, deterioration and the like type of trouble which leads to increases in noxious emission levels.